1. Field of the Invention
The present invention relates to a semiconductor device, particularly the structure of a semiconductor device suitable for preventing a soft error.
2. Description of the Prior Art
The term semiconductor device used herein collectively indicates a package, an LSI or IC chip mounted in the package and semiconductor elements, such as transistors and resistors, fabricated in the LSI or IC chip. Usually, the package is made of an insulating ceramic material and comprises a carrier, which is adapted to receive the LSI or IC chip, and a cover, which hermetically encloses the LSI or IC chip within the space of the carrier. The LSI or IC chip, hereinafter referred to simply as an LSI chip, is connected via wires to conductor leads formed on the carrier, which in turn are connected to an electronic device, such as a power source, located outside the semiconductor device.
A semiconductor device is described in, for example, U.S. Pat. No. 3,872,583.
It is known that the ceramic material of the package emits radioactive rays, particularly .alpha. rays which destroy memory data stored in the memory cells, i.e. one of the semiconductor elements of the LSI chip. The ceramic material of the package contains a few ppm of uranium and thorium, and the .alpha. rays are emitted in the radioactive disintegration of the uranium and thorium. When the .alpha. rays are transmitted through the memory cells formed at the surface of the semiconductor chip, a number of electron-hole pairs are generated in the memory cells, with the result that information memorized in the memory cells as existence or non existence of electrons or holes is falsified.
As in known, an error in information can occur due to the unfavourable influences of .beta. and .gamma. rays on the characteristics of the semiconductor elements, although the process of causing such information error due to the .beta. and .gamma. rays is different from that due to the .alpha. rays. .beta. rays are electrically charged particles and interact with semiconductor elements, and thus, increase a leak current of, for example, bipolar semiconductor elements. In addition, .gamma. rays cause the threshold voltage of MOSFETs, which are also semiconductor elements, to change.
The error of the information caused by the radioactive ray emission is referred herein to as a soft error. The soft error occurring during the operation of the semiconductor memory device, is detrimental to the reliability of the stored information.